Direct injection engines nominally operate at a higher fuel pressure (typically 5-12 Mpa) than other gasoline engines and the fuel injectors are accordingly sized for this higher pressure. When a direct injection engine is started, the fuel pressure is below the nominal value, and the pressure is limited to that supplied by a low pressure (typically electric) pump. Since the fuel pressure during engine starting is initially so much lower than the pressure available from the mechanical pump used for normal operation, long injector pulse widths are necessary to deliver the required fuel mass. Also, since the fuel is injected directly into the cylinder, the injection must be complete before the increasing cylinder pressure on the compression stroke exceeds the pressure in the fuel supply.
In typical engine control systems, an air bypass valve or electronic throttle is usually opened to a position that will allow sufficient air flow to quickly increase torque and allow the engine to reach an initial target high idle speed. This airflow helps maintain a higher intake manifold pressure during cranking and the initial start and allows the engine speed to rise rapidly once combustion is started. However, for the direct injection engine, a higher pressure in the manifold results in higher pressure in the cylinder and a smaller fraction of the compression stroke being available for fuel injection. Also, as the engine speed increases, the time available for fuel injection during any given portion of an engine cycle (typically the intake stroke and the first part of the compression stoke) is reduced. On the other hand, targeting a lower intake manifold pressure results in a smaller cylinder air charge, thereby requiring a correspondingly smaller mass of fuel to be injected at a given air/fuel ratio.